Cellular networks are increasingly used to connect to devices via wireless data links. Depending upon the cellular network, cellular communication can occur in one of number of frequency bands allocated for the purpose by the relevant regulatory authority. A device can be configured to communicate in multiple bands provided the device includes an antenna that has resonances in each of the bands.
A variety of different types of antennas are used in mobile applications including antennas that are external to the device such as helix and retractable antennas, and antennas that can be embedded within a device such as “inverted F” and folded dipole antennas. Embedding a device antenna within a mobile device case or housing is often preferable to prevent damage to the antenna and to reduce the form factor the mobile device. Embedded antennas can be constructed by printing metal circuit traces on a dielectric substrate of a printed circuit board (PCB). The resonance of such antennas typically depends upon the dimensions of the circuit traces and the dielectric constant of the PCB dielectric layers. As a general rule, the lower the resonant band of the antenna the larger the antenna.
A single antenna element can be used to transmit in multiple bands. However, wide-band operation of an antenna element typically sacrifices performance of the antenna elements and such wide-band operation is only practical for relatively closely spaced operating frequency bands. Therefore, operation at multiple frequency bands is typically supported using multiple antenna elements.
In a multiple-element antenna, different antenna elements are tuned for operation at different operating frequency bands. For example, suitably tuned separate antenna elements enable a multiple-element antenna to operate with Global System for Mobile Communications (GSM) and General Packet Radio Service (GPRS) and/or Code Division Multiple Access (CDMA) in the European and Asian frequency bands at approximately 900 MHz and 1800 MHz, or at the North American frequency bands at approximately 850 MHz and 1900 MHz.
Increasing integration is enabling the construction of small devices possessing high levels of functionality. For example, tracking devices including a GPS receiver and a microprocessor can now be constructed on a printed circuit board contained within a package having a size of 2.75″×2″×1″. As device form factors shrink, the size of a device's antenna can become a limiting factor. Reducing the size of an antenna typically reduces its efficiency. Therefore, reducing the size of an antenna in order to shrink a device's form factor can significantly impact the device's power consumption.